In chemical plants such as a methanol plant or an ammonia plant (including urea plant), high-temperature and high-pressure steam is used. FIG. 1 shows an example of the configuration of a steam system for controlling the steam.
The steam system 2 has a high-pressure header 4 which stores high-pressure steam therein and a low-pressure header 6 which stores low-pressure steam whose pressure is lower than that of the high-pressure steam. In some plants, a header corresponding to the low-pressure header 6 in FIG. 1 may be referred to as a medium-pressure side header.
The high-pressure header 4 is connected to a waste-heat boiler 8. The waste-heat boiler 8 supplies high-pressure steam to the high-pressure header 4. A supply system of the waste-heat boiler 8 has a safety valve 10 and a blow-off valve 12. When a steam pressure of the supply system exceeds a first predetermined pressure, a controller of the blow-off valve 12 gradually increases valve opening set to be full opened in normal time to release steam to the outside of the system. When the pressure of the supply system exceeds a second predetermined pressure set larger than the first predetermined pressure, the safety valve 10 is opened depending on the steam pressure to release steam to the outside of the system. The high-pressure header 4 is further connected to an auxiliary boiler system 14. The auxiliary boiler system 14 supplies high-pressure steam generated by an auxiliary boiler (package boiler) to the high-pressure header 4.
The low-pressure header 6 has a blow-off valve 30. When the steam pressure in the low-pressure header 6 exceeds a predetermined blow-off valve control start pressure, a controller 32 of the blow-off valve 30 gradually increases the valve opening set to be full opened in normal time to release steam to the outside of the system. This control is performed by means of a PI controller using a difference between a measurement value PV (Process Value) of the steam pressure in the low-pressure header 6 and a blow-off valve MV (Manipulated Value) set to be slightly larger than a target value of the steam pressure in the low-pressure header in normal time.
The low-pressure header 6 further has a safety valve 28. When the steam pressure exceeds a safety valve control start pressure set to be larger than the blow-off valve control start pressure, the safety valve 28 is opened depending on the steam pressure to release steam to the outside of the system. The low-pressure header 6 further supplies low-pressure steam to a low-pressure side system 34.
The high-pressure header 4 is connected to a turbine 16. High-pressure steam of the high-pressure header 4 is introduced into the turbine 16 through a turbine inlet piping 18. The turbine 16 is driven by the high-pressure steam, supplies mechanical energy to external apparatuses not shown and discharges steam with a lower pressure. Apart of the discharged steam is supplied to the low-pressure header 6 through a turbine outlet piping 20. Another part of the steam is supplied to a condenser not shown and the like.
The steam system 2 further has a turbine bypass line 22 connecting the high-pressure header 4 to the low-pressure header 6. The turbine bypass line 22 has the turbine bypass valve 23 for controlling a flow of steam flowing therein. When the turbine bypass valve 23 is opened, high-pressure steam of the high-pressure header 4 is supplied to the low-pressure header 6 through the turbine bypass line 22.
The turbine bypass valve 23 is controlled by operating a solenoid according to a control signal sent from a control part 24. The control part 24 has a high-pressure side controller 25, a low-pressure side controller 27 and a higher-order selector 26.
The high-pressure side controller 25 receives an input of a high-pressure side pressure being a value obtained by measuring pressure in the high-pressure steam of the high-pressure header 4. Based on a pre-stored process, the high-pressure side controller 25 generates high-pressure side MV for instructing opening of the turbine bypass valve 23 from the input high-pressure side pressure and outputs the high-pressure side MV.
The low-pressure side controller 25 receives an input of a low-pressure side pressure being a value obtained by measuring pressure in the low-pressure steam of the low-pressure header 6. Based on a pre-stored process, the low-pressure side controller 25 generates low-pressure side MV for instructing opening of the turbine bypass valve 23 from the input low-pressure side pressure and outputs the low-pressure side MV.
The higher-order selector 26 receives inputs of the high-pressure side MV and the low-pressure side MV, selects a larger value of them as MV for controlling the turbine bypass valve 23 and sends steam of controlled amount from the high-pressure header 4 to the low-pressure header 6. According to such control, when steam pressure in the high-pressure header 4 becomes higher than a predetermined level, the steam pressure in the high-pressure header 4 can be decreased. Furthermore, when the steam pressure in the low-pressure header 6 becomes lower than a predetermined level, the steam pressure in the low-pressure header 6 can be increased.
The low-pressure header 6 is further connected to a low-pressure steam supply system not shown. The low-pressure steam supply system supplies low-pressure steam to the low-pressure header 6. The low-pressure steam supply system is controlled by a control device which previously stores low-pressure side flow control SV (Set Value) therein. When pressure in the low-pressure header 6 exceeds the low-pressure side flow control SV, the amount of steam supplied from the low-pressure steam supply system to the low-pressure header 6 is decreased.
Japanese Laid-Open Patent Application JP-A-Heisei, 11-257018 describes an invention concerning a steam turbine steam bypass device for smoothly releasing steam used on a turbine side to a high-pressure steam condenser when the steam turbine is shut down in an emergency due to break-down (at trip).
Japanese Laid-Open Patent Application JP-A-Heisei, 7-229405 describes a turbine bypass control method in a combined plant including: a turbine bypass connected to an inlet of a steam turbine and having a turbine bypass valve; and a turbine governor for controlling the turbine bypass valve, wherein, when the turbine governor stops an automatic control of the turbine bypass valve, the turbine governor controls the turbine bypass valve using pressure higher than the steam pressure at this time by a predetermined value as a set pressure.